1. Field of the Invention
This invention relates to cylindrical electronically scanned antenna systems which scan at rates faster than the information being processed and more particularly to improvements in the signal combining subsystem of such systems to simultaneously achieve high values of directive gain and separation of multiple signals based on frequency, time-of-arrival and direction-of-arrival.
2. Description of the Related Art
It is sometimes desirable to configure a system to receive all of the electromagnetic signals within the receiver's capabilities as limited by its sensitivity and bandwidth. Signals of interest are usually incident from widely diverse directions. Therefore, prior systems have utilized antennas having a wide azimuth beam width, such as omnidirectional antennas, as the system's receptor.
A severe limitation of this approach is that it does not permit directional resolution of multiple signals. Such resolution is usually desirable to prevent garbling of signals that cannot otherwise be resolved in frequency or time-of-occurrence. Directional resolution is also desirable in cases where the direction of incidence of the signals is to be estimated.
To overcome these disadvantages, alternative prior art systems have been configured using narrow-beam antennas. In one case, multiple antennas, each producing a narrow beam, are arranged in a circular pattern so that their beams are contiguous and point radially outward. In another case, a single cylindrical array antenna is configured to form multiple beams which are contiguous and point radially outward. In both cases, each beam port of the antenna(s) is connected to a separate receiver, thus the system can exhibit the advantages of both good directional resolution and complete, simultaneous directional coverage. However, the disadvantage in this case is the high cost of the multiple receivers.
Another class of prior art systems attempts to achieve omnidirectional coverage with a single narrow beam by scanning that beam as a function of time. In these systems, a narrow beam is scanned over all azimuths by mechanical rotation of a fixed-beam antenna, or by electronic scan of a cylindrical array antenna. The disadvantage in this case is that the beam cannot look everywhere at once.. This is especially a problem for multiple signals from diverse directions if they are nonrepetitive in character or have rapidly changing wave forms (high information rate or short-pulse signals). These high information rate signals may not be sampled at a sufficient rate by the scanning beam to prevent information loss.
More recently, techniques have been disclosed which address the problems associated with directional resolution of multiple signals. Our patent application, U.S. patent application Ser. No. 719,460, filed Nov. 20 1984, provided a cylindrical array antenna system capable of scanning a narrow beam through its complete coverage sector at a rate at least twice as fast as the maximum information rate of the signals it receives so that no information is lost. This allows the system to scan within the time period of the shortest pulse which it is expected to receive and thereby have a high probability of intercepting and receiving that signal. This system provided angular resolution of multiple signals and the capabilities of determining their direction of arrival commensurate with the narrow beam widths of a full N element cylindrical array. The system provided the same sensitivity and angular resolution regardless of the direction of signal incidence. These improvements were the result of using heterodyne techniques to achieve very rapid scanning of a single beam throughout the antenna's entire sector of coverage.
This technique, however, does result in a sensitivity loss due to sampling. This loss occurs because the scanning beam is only directed at the angle of incidence for a short period of time during a scan. The scanning beam will intercept the incident signal for only 1/Nth of the scanning period. The sampling loss in dB is given by 10 log N. This degrades the sensitivity to that of a single element of the array or less.
Our more recent patent application, U.S. patent application Ser. No. 807,871, filed Oct. 28, 1985, describes an antenna system which provides multiple scanning beams to eliminate the sampling loss of the prior art. Our still more recent patent application, U.S. application Ser. No. 899,629, herein incorporated by reference filed Aug. 21 , 1986 describes an improved antenna system which covers wider bandwidth than that described in U.S. patent application Ser. No. 807,871 and also provides the ability to resolve multiple signals by their time-of-arrival, direction-of-arrival and frequency. Most recently, our patent application U.S. patent application Ser. No. 011,051, filed Dec. 29, 1986, describes another wideband antenna system which is more hardware efficient than that described in U.S. patent application Ser. No. 899,629, but which does not resolve multiple signals by their frequency. The present invention also creates multiple scanning beams which are used in a wide bandwidth arrangement to eliminate the sampling loss of the prior art. The present invention, however, is distinguished by its use of a different arrangement to provide fewer system outputs, rendering the present invention more practical than the art of U.S. patent application Ser. No. 899,629, while retaining most of the frequency resolution capability of this prior art.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a cylindrical array antenna system capable of scanning through its complete coverage sector, at least twice as fast as the maximum information rate of the signals it receives so that no information is lost. This capability will allow the antenna to scan within the time period of the shortest pulse which it is expected to receive and thereby the antenna will have the same high probability of intercepting that signal as could be achieved with an omnidirectional antenna.
It is another object of the present invention to achieve the gain available from the full cylindrical array by recovering the signal loss which occurs in the cylindrical arrays of other systems due to sampling the signal during only a portion of its time of presence,
Another object of the invention is to provide multidimensional resolution of multiple incident signals, sorting them by their time-of-arrival, direction-of-arrival and frequency.
It is yet another object of the present invention to provide the ability to determine the frequencies of multiple incident signals and the ability to determine their direction-of-arrival commensurate with the narrow beamwidth achievable with a full N element cylindrical array.
It is another object of the present invention to receive incident signals with the same sensitivity and resolution regardless of the direction of signal incidence.
Another object of the present invention is to gain the advantages of the above objects using fewer components than are required for multiple beam antenna systems that require separate complete receivers for each beam.
Another object of the present invention is to gain the advantages of the above objects using fewer outputs than required for earlier cylindrical array antenna systems such as the invention of U.S. patent application Ser. No. 899,629.
In general, the present invention, an apparatus for adding the capability to separate multiple signals and for eliminating the sampling loss of signal energy in systems having a coverage sector through which the antenna system scans multiple beams at a rate that is faster than the information rate being received, comprises:
(a) a cylindrical phased array antenna comprising a plurality of radiator elements evenly spaced around a circular arc;
(b) means for decomposing the distribution of current on the radiator elements caused by electromagnetic wave incidence into component signals which are the Fourier spatial harmonics of the distribution;
(c) means for forming a plurality of beams of sensitivity from said component signals, said plurality of beams of sensitivity being equal in number to the number of antenna elements in said circular arc, the beams being contiguous and considered as lying in the azimuth plane for reference purposes, with each beam being generally evenly spaced from adjacent beams in .theta. space, where .theta. is the angle away from boresight in the azimuthal plane, the spacing between beam center directions in .theta. space being generally proportional to the reciprocal of the number of antenna elements, and the beams, taken together to form a larger composite beam, span the entire azimuth coverage sector;
(d) means for differentially weighting the amplitude of said component signals to achieve a desired time invariant relative weighting of the signals for beam shape control;
(e) means for differentially delaying and phase shifting said component signals to achieve a desired time invariant relative phasing of the signals for beam focusing;
(f) means for differentially phase shifting these component signals at rates exceeding 4 .pi. radians per cycle of the highest frequency present in the information content of the incident electromagnetic wave for synchronously scanning each of the beams over the entire coverage sector, the beams maintaining their relative positions adjacent one another in .pi. space during scanning, the scanning being carried out periodically at a rate that is at least twice as fast as the highest information rate being received;
(g) means for accepting signals received by each beam and differentially delaying said beam signals to cause their modulation envelopes to respond in unison to a single emitting source at a particular azimuth angle;
(h) means for coherently combining said beam signals after said signals have been differentially delayed, said means for combining being arranged to cause coherent addition selectively at different output ports of said combiner for different signal frequencies; and
(i) means for measuring an approximate frequency of said combined beam signals for eliminating ambiguities in determination of signal frequency.